The present invention pertains to preventing unauthorized access and/or copying of compressed video data.
Herein, the term encoded video signal is used to refer to a video signal that is compressed and formatted according to a predefined syntax and semantics of a well-known standard, such as MPEG-2. The term decoded video signal is used to refer to a video signal that has been deformatted, and decompressed, according to such a well-known standard, e.g., for presentation or display. The term scrambling is used to refer to a technique of rendering a video signal unintelligible using a not well known or secret formula or methodology. The term descrambling is used to refer to the inverse technique to scrambling that allows recovery of the video signal using the secret methodology. The term encryption refers to a particular manner of scrambling, for example, whereby a signal is rendered unintelligible using a key. The term decryption refers to an inverse technique to encrypting that allows recovery of the video signal, for example, using a key. Both private and public key encryption techniques are known. Herein, the terms xe2x80x9cencryptionxe2x80x9d and xe2x80x9cdecryptionxe2x80x9d are used to refer to a higher complexity processing than xe2x80x9cscramblingxe2x80x9d and xe2x80x9cdescramblingxe2x80x9d which (encryption, scrambling, decryption and descrambling) processings render information unintelligible or recover information from its unintelligible form.
A pervasive problem for distributors and broadcasters of video programs is the unauthorized access, e.g., viewing or copying, of the video programs. Both a broadcast service provider (such as a cable television or direct broadcast satellite service provider) and a content provider (such as a distributor of packaged media, e.g., video tapes and optical discs) desires to restrict access to video program under a variety of circumstances such as:
(1) restricting access by younger viewers to adult-oriented subject matter video programs,
(2) restricting access to video program events (e.g., pay-per-view or rental video shows) or entire video program services (e.g., premium video program services) to only subscribers who pay for them, and
(3) restricting access to video program events and video program services to subscribers in certain geographic regions.
The prior art has suggested several techniques for restricting unauthorized access to video programs including scrambling or encrypting the video program material. Authorized access is achieved by descrambling the video program material or by decrypting it with a key (wherein a public key encryption technique may be used). Good encryption techniques tend to thwart unauthorized access but generally impose a heavy penalty on the authorization system. Specifically, the decrypting process tends to be highly computationally intensive and therefore requires specialized high processing capability hardware.
It is an object of the present invention to reduce the computation requirements for restricting access to encoded video programs yet still provide a high level of protection against unauthorized access.
This and other objects are achieved by the present invention. According to one embodiment, a method and apparatus are provided for restricting access to a digital video signal. According to the method, the digital video signal is encoded to produce an encoded video signal. In encoding the digital video signal, motion compensated encoding is performed on one or more first video picture portions of the digital video signal using a second video picture portion of the video signal as a reference for forming predictions therefor. Only the second video picture portion of the encoded video signal is scrambled.
Illustratively, the second video picture portion is scrambled using a first function of a first information. The first information is then encrypted using a second function of higher computation complexity than the first function. The encrypted first information is then inserted into the encoded video signal.
Illustratively, the second video picture portion originates in an intracoded (I) or predictively encoded (P) picture which serves as a reference picture.
As may be appreciated, scrambling is preferably achieved using a simple (first) function such as XOR"" ing the video picture data using a pseudo random sequence generated from a seed (first information). The seed can be encrypted using a complicated (second) encryption function which is difficult to discover without afore-knowledge of the particular second function used. Because only a limited amount of information is encrypted or scrambled, the processing needed for encoding or decoding the video signal is less than in the prior art.
Moreover, an additional savings is achieved by scrambling information in reference pictures. In particular, reference pictures are used to form predictions for motion compensated decoding of other pictures. By scrambling the information in the reference pictures used for forming predictions, it is not possible to obtain predictions for motion compensated decoding other pictures to produce intelligible video. Simply stated, the encoding process causes information in certain pictures (namely, reference pictures) to be more significant than others, and according to this invention, it is only this significant information which is scrambled.
According to another embodiment, a method and apparatus for enabling access to a video signal are provided. According to the method, The encoded video signal is received. Only a first video picture portion of the video signal is descrambled. The encoded video signal is then decoded. In decoding the encoded video signal, motion compensated decoding is performed on one or more second video picture portions of the video signal using predictions formed from the descrambled first video picture portion.
Illustratively, an encrypted version of the first information is retrieved from the encoded video signal. The first information is decrypted using a first function. The descrambling of the first video picture portion is descrambled using a second function of the decrypted information having a lower computation complexity than the first function.